From the viewpoint of simple and inexpensive image production, inkjet recording has been used in a variety of printing fields. As the inkjet recording method, there is an actinic radiation-curable inkjet method in which droplets of UV-curable ink are landed on a recording medium and then the ink is cured by irradiation with actinic radiation to form an image. Recently, the actinic radiation-curable inkjet method has been attracting attention because it provides high adhesiveness even when being applied on recording media which lack ink absorbing properties or when being applied on another ink for top coating.
In relation to an actinic radiation-curable inkjet ink, there is known a technique of allowing an actinic radiation-curable inkjet ink to contain a wax or a gelling agent in order to prevent inks from being unnecessarily mixed together and to further enhance the rub resistance of an image to be obtained. In this technique, the wax or the gelling agent enables sol-gel phase transition through a temperature change, and thus the ink undergoes gelation when being attached onto a recording medium so that the mixing of inks and the spreading of dots can be prevented. In addition, the viscosity of an ink can be increased even at low temperatures, which provides a printed article further having excellent rub resistance at room temperature.
PTL 1 and PTL 2 each describe an image forming method in which a color ink and a white ink are used in combination to produce a desired esthetic performance. In this case, use of an actinic radiation-curable inkjet ink capable of sol-gel phase transition through a temperature change for the white ink enables formation of an image in an easier manner. Further, a background color can be formed by using the white ink for top coating or primer coating in formation of an image with other inks.
For a colorant in a white ink, pigments such as titanium dioxide are used. Titanium dioxide is poor in light resistance as it is, and thus is used with the surface covered with alumina or the like. However, the high tendency of alumina to polarize makes it difficult for alumina to disperse stably in a non-polar solvent such as one to be used for an actinic radiation-curable inkjet ink, and the particles easily associate together. Accordingly, inks containing titanium dioxide as a pigment are allowed to contain a dispersant to enhance the dispersibility of the pigment for stable storage of the ink. PTL 3 describes, for example, an ink containing a white pigment such as titanium dioxide in which the pigment is dispersed in a dispersant containing a copolymer or block copolymer having a pigment-affinic group. As such dispersants, dispersants having a secondary amine as a pigment-affinic group are disclosed. As the gelling agent is used a free radical-curable gelling agent obtained by polymerization via amide bonds.
Alternatively, titanium dioxide can be used to inhibit the blooming of an ink. PTL 4 describes an actinic radiation-curable inkjet ink which is capable of gelation through a wax and contains, as an inorganic fine particle having a weak coloring ability, less than 5% of titanium dioxide relative to the mass of the ink.